Schellhaas, U. | Search bioRxiv

bioRxiv2025

Structure of cytoplasmic RNA polymerase II

RNA polymerase II (Pol II) must be assembled in the cytoplasm before it enters the nucleus, where it transcribes protein-coding genes. Although transcription by Pol II is intensively studied, how this central multi-subunit enzyme is made and the role of dedicated factors remains unclear. Here, we report the integrative structural analysis of a native human Pol II from the cytoplasm captured near the end of biogenesis. The complex contained Gdown1 and three biogenesis factors - RPAP2 and the critical small GTPases GPN1 and GPN3. Cryo-EM analysis of the complex revealed how Gdown1 and RPAP2 associate with Pol II and prevent the premature association of transcription factors. Further biochemical and cryo-EM analysis revealed how RPAP2 recruits GPN1-GPN3 to the complex, and how the assembly of the RPAP2-GPN1-GPN3 complex is controlled by GTP hydrolysis. The combined results uncover a network of interactions that chaperone cytoplasmic Pol II to prevent aberrant interactions, reveal a GTP-controlled switch during the final stages of Pol II biogenesis, and suggest a general mechanism for the action of GPN-loop GTPase family of enzymes.

biochemistry↗

bioRxiv2024

A molecular switch orchestrates the nuclear export of human messenger RNA

The nuclear export of messenger RNA (mRNA) is a key step in eukaryotic gene expression (Kohler and Hurt, 2007). Despite recent insights into the packaging of newly transcribed mRNAs into ribonucleoprotein complexes (mRNPs) (Pacheco-Fiallos et al., 2023; Bonneau et al., 2023), the subsequent events that govern mRNA export are poorly understood. Here, we elucidate the molecular basis of human mRNA export licensing, which involves the remodeling of mRNP-bound transcription-export complexes (TREX), the formation of export-competent mRNPs, the docking of mRNPs at the nuclear pore complex (NPC), and the release of mRNPs at the NPC to initiate export. Our biochemical and structural data uncover the ATPase DDX39/UAP56 as a central molecular switch that directs mRNPs through the TREX and the NPC-anchored TREX-2 complexes using its ATPase and mRNA-binding cycle. Collectively, these findings establish a mechanistic framework for a general and conserved mRNA export pathway.

molecular biology↗